1. Field of the Invention
The present invention relates to a functional device based on the MEMS (Micro Electro Mechanical Systems). The MEMS is used for manufacturing high-frequency filters, various sensors, actuators, and the like by etching a substrate including Si as a main component.
2. Description of the Related Art
In the field of MEMS, functional devices such as high-frequency sensors, acceleration sensors, angular velocity sensors, and actuators are manufactured by processing a base material including Si as a main component using the etching technique. The base material may comprise a Si wafer or a wafer including an insulating layer (the wafer is called as SOI (Silicon on Insulator)).
A SAW (Surface Acoustic Wave) filter is typically used for a typical high-frequency filter used for cellular phone and the like. Recently, a FBAR (Film Bulk Acoustic Resonator) filter is also used. When the SAW filter is manufactured, a functional device is formed on a substrate of a piezoelectric element, while a functional device in the FBAR filter is manufactured by forming a piezoelectric membrane on a substrate including Si as a main component. To ensure property and reliability of the functional device in these high-frequency filters, the function element section needs to be sealed air-tight.
In sensors for acceleration based on the MEMS, a first method of forming a functional element is to measure acceleration loaded to an entire device from a change in movement of the functional device. In this case, a spindle-shaped or comb-like electrode is used through etching. In a second method, acceleration is read from a change in resistance generated because of distortion loaded to a resistor previously formed on a beam. In this case, a through-hole is provided by etching a substrate containing Si as a main component, and a spindle is held via a thin beam to form a resistor on the beam. Even in either method is adopted, because movement of an electrode or a spindle is influenced by an air pressure within a package, air-tight sealing for the package is required. Furthermore there has been developed, as a third method, a method for detecting acceleration by measuring temperature distribution within the functional device with a temperature sensor. In this case, a hollow structure is formed on a substrate containing Si as a main component, and a thin, net-like beam is formed on the hollow structure. A temperature sensor is formed at a prespecified position on the beam, while a heat source is formed on a portion of the beam. Thus, in the state where a gas inside the package is heated by the heat source, when acceleration is loaded, the gas flows in the package. By measuring the temperature distribution in such a state, the loaded acceleration is detected. Also in the functional device having the configuration as described above, air-tight sealing is required for the package.
In any case, air-tight sealing for the package is indispensable for maintaining the performance in sensors having a movable section like the MEMS or a high-frequency filter.
Examples of a conventional technique for providing air-tight sealing include a method in which a functional device is provided on a ceramic package and a cover made of metal, ceramics, glass or the like is connected to the functional device with solder or glass having a lower melting point. In the methods, however, the functional device is packaged discretely.
Recently, contemplation of completing air-tight sealing for the MEMS functional device in a wafer state has been made as a method of reducing the packaging cost for the MEMS-based functional device. To achieve the objective, it is necessary to joint another water to a substrate containing Si as a main component with a functional device formed thereon and seal the jointed portion air-tight. Such an example is described, for instance, in DIGEST OF TECHNICAL PAPERS 2002:IEEE International Solid-State Circuits Conference 0-7803-7335-9/02/2002 IEEE (Non-patent document 1)